The hypothetical but strongly evidenced common ancestor of the
Indo-European languages. PIE words are reconstructed from extant
Indo-European languages. There is no clear agreement on exactly where
or when the speakers of PIE lived. It is believed that most of the
subgroups diverged and spread out over much of Europe, Iran, and
northern Indian subcontinent during the fourth and third millennia BC.
See also → proto-language.

The hypothetical and typically extinct language which is believed to be the
ancestor of a group of languages of the same family.
Historical linguistics uses comparative study of the languages of a family
to reconstruct the ancestral language even though in most cases it was
never recorded. Some examples are → Proto-Indo-European,
Proto-Germanic, Proto-Romance, Proto-Sino-Tibetan, etc.